CVE-2026-49842: FreeSWITCH DoS Vulnerability

CVE-2026-49842 Overview

CVE-2026-49842 is a denial-of-service vulnerability in FreeSWITCH, the open-source Software Defined Telecom Stack. The flaw resides in mod_verto's WebSocket frame loop, which intercepts a #-prefixed speed-test protocol before performing authentication. Unauthenticated remote attackers can request payloads up to INT_MAX bytes through the #SPU command. The server responds with approximately ten times the requested size, generating roughly 20 GB of outbound traffic per request. This bandwidth amplification primitive allows attackers to exhaust network resources from a short, unauthenticated request. The issue is fixed in FreeSWITCH version 1.11.1.

Critical Impact
Unauthenticated attackers can trigger ~20 GB outbound responses per request, enabling network amplification and denial-of-service against FreeSWITCH deployments exposing mod_verto WebSockets.

Affected Products

FreeSWITCH versions prior to 1.11.1
Deployments with mod_verto enabled and exposed via WebSocket
Any commodity hardware running affected FreeSWITCH builds

Discovery Timeline

2026-06-09 - CVE-2026-49842 published to NVD
2026-06-10 - Last updated in NVD database

Technical Details for CVE-2026-49842

Vulnerability Analysis

The vulnerability stems from mod_verto's WebSocket frame handler processing speed-test commands before authentication. When a client sends a frame beginning with #SPU, #SPB, or #SPE, the module dispatches the request directly to the speed-test routine. No session credentials or peer validation gate this code path.

The #SPU command declares the payload size as an integer parsed with atoi(). The parser only rejects non-positive values, allowing attackers to supply sizes up to INT_MAX (2,147,483,647 bytes). During the download phase, the server writes approximately size * 10 bytes back to the peer, producing roughly 20 GB of outbound traffic per request. This is classified as Uncontrolled Resource Consumption [CWE-400].

Root Cause

The root cause is twofold. First, authentication ordering is incorrect: the speed-test handler executes before the session is authenticated. Second, input validation on the declared payload size is insufficient. The use of atoi() without bounds checking accepts any positive 32-bit integer as a legitimate request size.

Attack Vector

An attacker establishes a WebSocket connection to a FreeSWITCH host running mod_verto. The attacker sends a #SPU frame declaring a large payload size, for example near INT_MAX. The server immediately begins streaming the multiplied payload back. Repeated or parallel requests yield strong amplification, enabling outbound bandwidth saturation or reflection scenarios where the attacker spoofs or proxies the target.

No authentication, user interaction, or special privileges are required. The attack is reachable over any network path that can reach the Verto WebSocket port.

Detection Methods for CVE-2026-49842

Indicators of Compromise

Inbound WebSocket frames to mod_verto containing #SPU, #SPB, or #SPE prefixes from unauthenticated peers
Sustained outbound traffic bursts from FreeSWITCH hosts in the multi-gigabyte range correlated with short inbound requests
Connection patterns where session teardown follows immediately after large download bursts

Detection Strategies

Inspect FreeSWITCH logs for mod_verto speed-test invocations occurring prior to authenticated session establishment
Deploy network monitoring to flag asymmetric byte ratios between FreeSWITCH inbound and outbound traffic exceeding normal Verto session profiles
Apply WebSocket-aware deep packet inspection to identify #SP prefixed frames originating from untrusted sources

Monitoring Recommendations

Track per-source connection rates and cumulative outbound bytes for Verto endpoints
Alert on FreeSWITCH process egress traffic that deviates from historical baselines
Correlate firewall flow data with mod_verto connection logs to identify unauthenticated amplification attempts

How to Mitigate CVE-2026-49842

Immediate Actions Required

Upgrade FreeSWITCH to version 1.11.1 or later, where the speed-test handler enforces authentication and bounds-checks payload sizes
Restrict network access to the mod_verto WebSocket port to trusted client networks via firewall rules
Disable mod_verto on FreeSWITCH instances that do not require Verto WebRTC signaling

Patch Information

The vendor released a fix in FreeSWITCH 1.11.1. The patch is described in the GitHub Security Advisory GHSA-p3gx-p2w7-wp35 and shipped as part of the FreeSWITCH v1.11.1 release. Operators should validate the upgrade against existing Verto integrations before production rollout.

Workarounds

Place mod_verto behind an authenticating reverse proxy that drops frames starting with #SP from unauthenticated clients
Apply per-connection egress rate limiting on the FreeSWITCH host to cap amplification potential
Remove load mod_verto from modules.conf.xml if Verto functionality is unused

bash

# Disable mod_verto in FreeSWITCH configuration
sed -i 's|<load module="mod_verto"/>|<!-- <load module="mod_verto"/> -->|' \
  /usr/local/freeswitch/conf/autoload_configs/modules.conf.xml

# Restrict Verto WebSocket port to trusted networks
iptables -A INPUT -p tcp --dport 8081 -s 10.0.0.0/8 -j ACCEPT
iptables -A INPUT -p tcp --dport 8081 -j DROP